Bound CO Is A Molecular Probe of Electrostatic Potential in the Distal Pocket of Myoglobin

Abstract

Most recent experiments have indicated that distal pocket polarity rather than steric hindrance is the major factor governing the distribution of FeCO stretching frequencies (ν C-O , ν Fe-CO ) in myoglobins and hemoglobins. Hydrogen bonding and other polar interactions have also been shown to play a key role in regulating O 2 and CO binding. To quantify the effects of polarity on ν C-O , ν Fe-CO , and ligand binding, we calculated electrostatic potential field distributions in the distal pockets of 18 different mutants and two wild-type forms of recombinant pig and sperm whale MbCO. The results were obtained using linearized Poisson-Boltzmann methods with coordinates from high-resolution structures determined experimentally by X-ray crystallography. The computed potential fields at the ligand atoms vary from +30 to -12 kcal/mol depending on the protein structure at the distal site. The electrostatic fields correlate inversely with ν C-O and directly with ν Fe-CO . In all our calculations, the distal histidine is modeled as the neutral N -H tautomer, regardless of which ferrous ligand is bound. If the neutral Nδ-H tautomer is used, the computed potentials at the bound ligand atoms are uniformly negative and show no correlation with ν C-O , ν Fe-C , and any ligand binding parameters. Although calculated using primarily MbCO structures, there is a linear, inverse relationship between the electrostatic field at the ligand binding site and the logarithm of the rate constant for O 2 dissociation. As a result, high O 2 affinity can be predicted semiquantitatively from a large positive potential field or from an experimentally low value of ν C-O . Thus, the stretching frequency of bound CO serves as an empirical voltmeter that can be used to measure the polarity of the distal pocket and to predict the extent of electrostatic stabilization of bound O 2 .

Crystal Structure of the Dioxygen-bound Heme Oxygenase from Corynebacterium diphtheriae

“…Using mutagenesis, Wilkinson, Phillips, Olson, and their colleagues have demonstrated unambiguously that increasing the strength and number of hydrogen bonds donated from the distal pocket amino acids to bound O 2 lowers the rate of oxygen dissociation from Mb (39,41,55,56). The best example is V68N pig Mb, in which Val at the E11 helical position is replaced with Asn.…”

Probing dynamics of complex molecular systems with ultrafast 2D IR vibrational echo spectroscopy

“…Carbonmonoxy-myoglobin (MbCO) has been extensively studied both experimentally [47][48][49][50][51][52][53][54][55][56] and computationally. [57][58][59][60] Myoglobin (Mb) is a small globular heme protein weighing approximately 17 kDa that is found in mammalian muscle tissue.…”

Ultrafast Dynamics of Myoglobin without the Distal Histidine: Stimulated Vibrational Echo Experiments and Molecular Dynamics Simulations

“…5,8,10,11 A wealth of experimental and theoretical evidence underscores the importance of H64 in myoglobin function and dynamics. 23,[30][31][32][33] H64 is the most polar residue near the active site, and interacts strongly with both CO and O 2 ligands. H64 has been implicated in the physiologically important differentiation between CO and O 2 binding to the heme group of myoglobin and hemoglobin.…”